Since carbon has a property that it is hardly wetted with a melt such as a slag, carbon-contained refractories have an excellent durability. Accordingly, in recent years, they have been widely used as lining refractories of various molten metal containers. For example, when magnesia is used as a refractory filler, an excellent durability is exhibited as lining refractories of molten metal containers because of the property provided by carbon and a corrosion resistance to melt provided by magnesia.
However, as carbon-contained refractories have been increasingly used, elution of carbon of refractories in molten steel which is so-called carbon pickup has been problematic. Especially, in recent years, high-quality steel has been required more severely, and refractories having a lower carbon content has been in high demand. Meanwhile, from the aspect of inhibition of heat dissipation from containers or environmental protection such as energy saving, the use of refractories having a low thermal conductivity has been required. From this standpoint as well, refractories having a low carbon content has been demanded.
As carbonaceous raw materials used in carbon-contained refractories, flake graphite, a pitch, a coke, mesocarbon and the like have been so far mainly used. For obtaining refractories having a low carbon content, the mere reduction of the use amount of these carbonaceous raw materials has involved a problem of the decrease in thermal shock resistance. In order to solve this problem, official gazette of JP-A-5-301772 proposes refractories in which expanded graphite is used as a carbonaceous raw material. Examples thereof describe a magnesia carbon brick obtained by kneading a refractory raw material composition comprising 95 parts by weight of sintered magnesia, 5 parts by weight of expanded graphite and 3 parts by weight of a phenol resin, press-molding the composition and then heat-treating the molded product at 300° C. for 10 hours. It is described that a spalling resistance is improved in comparison to the use of the same amount of flake graphite.
Official gazette of JP-A-11-322405 discloses carbon-contained refractories having a low carbon content, characterized in that in a raw material blend comprising a refractory raw material and a carbonaceous raw material containing carbon, a fixed carbon content of the carbonaceous raw material is from 0.2 to 5% by weight per 100% by weight of a hot residue of the raw material blend and carbon black is used in at least a part of the carbonaceous raw material (claim 5). In the official gazette, it is explained that since carbon black has a small grain size of approximately 0.1 μm, a dispersibility in a refractory texture is significantly high, surfaces of filler grains can be coated with fine carbon grains, and the contact of filler grains can be blocked even at a high temperature over a long period of time to inhibit excessive sintering. Examples describe refractories formed by molding a raw material blend obtained by blending a refractory filler comprising 50 parts by weight of magnesia and 50 parts by weight of alumina with 2.5 parts by weight of a phenol resin, 1 part by weight of a pitch and 1 part by weight of carbon black (thermal) and baking the molded product at from 120 to 400° C., indicating that the refractories are excellent in spalling resistance and resistance to oxidative damage.
Official gazette of JP-A-2000-86334 describes a brick for a sliding nozzle apparatus obtained by adding from 0.1 to 10% by weight, based on outer percentage, of carbon black having a specific surface area of 24 m2/g or less to a blend comprising a refractory filler and a metal, further adding an organic binder, kneading the mixture, molding the resulting mixture and then heat-treating the molded product at a temperature of from 150 to 1,000° C. It is indicated that the incorporation of specific carbon black (thermal class or thermal black class) in a spherical form having a large grain size of from 80 to 500 nm provides a good packing property and a dense brick texture to decrease a porosity and used carbon black itself is also excellent in oxidation resistance, whereby refractories excellent in oxidation resistance are obtained. Examples describe refractories obtained by molding a blend comprising 97 parts by weight of alumina, 3 parts by weight of aluminum, 3 parts by weight of a phenol resin, 3 parts by weight of a silicon resin and 3 parts by weight of carbon black and heating the molded product at a temperature of 500° C. or less, indicating that the refractories are excellent in oxidation resistance.
Official gazette of JP-A-7-17773 describes monolithic refractories in which from 0.1 to 3% by weight of spherical carbon black having a large grain size of from 0.02 to 0.50 μm and imperfect in structure development is added to a refractory filler. Further, official gazette of JP-A-10-36177 describes a blast furnace taphole mud containing from 2 to 15% by weight of carbon black having a DBP absorption of 100 ml/100 g or less and fixed amounts of a carbonaceous raw material, silicon carbide, silicon nitride, a refractory raw material and a carbon-containing binder. Still further, official gazette of JP-A-2000-192120 describes a taphole mud comprising a refractory filler, carbon black having a DBP absorption of from 15 to 80 ml/100 g, a pitch and a binder.
On the other hand, official gazette of JP-A-2000-273351 discloses a process for producing graphitized carbon black, which comprises heat-treating a mixture containing carbon black and a graphitization-promoting substance at from 2,000 to 2,500° C. The temperature of approximately 2,800° C. so far required for graphitization of carbon black can be reduced to from 2,000 to 2,500° C. by heating along with a graphitization-promoting substance made of an element such as boron, silicon, aluminum or iron or its compound.
However, as described in JP-A-5-301772, the use of expanded graphite as a carbonaceous raw material can provide a good thermal shock resistance even in low-carbon refractories in which the use amount thereof is approximately 5% by weight as compared to the use of flake graphite in the same amount. Nevertheless, expanded graphite is a highly bulky raw material. Accordingly, even when the use amount is as small as approximately 5% by weight, a packing property of refractories is decreased, and a corrosion resistance to melt is poor. Moreover, the oxidative loss of the carbonaceous raw material during use of refractories was also a serious problem.
Official gazettes' of JP-A-11-322405, JP-A-2000-86334, JP-A-7-17773, JP-A-10-36177 and JP-A-2000-192120 all describe examples of using carbon black as a carbonaceous raw material. Although the employment of carbon black was deemed to improve a spalling resistance, a corrosion resistance and an oxidation resistance were still insufficient. Further, carbon black used includes carbon black having a specific surface area of less than 24 m2/g, spherical carbon black having a large grain size and imperfect in structure development and carbon black having a DBP absorption of less than 100 ml/100 g or from 15 to 80 ml/100 g. That is, carbon black having a large grain size with a low DBP (dibutyl phthalate) absorption is deemed to be rather preferable. The employment of such a carbon black was, however, still insufficient to improve a thermal shock resistance.
Further, for forming a dense texture or improving an oxidation resistance, a method in which a powder of a single substance of aluminum, silicon, magnesium or the like or a powder of a compound except an oxide, such as boron carbide or silicon carbide, was mainly employed. In this method, however, for obtaining sufficient effects, these additives had to be used in large amounts, which had, in many cases, an adverse effect on other characteristics consequently. For this reason, there was no choice but to make a compromise at some level.
Official gazette of JP-A-2000-273351 describes a process in which carbon black and a graphitization-promoting substance such as boron are heat-treated for graphitization. However, it is used in a carrier for a catalyst of a phosphoric acid-type fuel cell, and there is nothing to describe or suggest that such a graphitized carbon black is useful as a raw material of refractories.
The invention has been made for solving the foregoing problems. It is an object of the invention to provide refractories excellent in corrosion resistance, oxidation resistance and thermal shock resistance, especially, carbon-contained refractories having a low carbon content. Such carbon-contained refractories having a low carbon content are useful because carbon pickup in molten steel is reduced and heat dissipation from containers is decreased. Another object of the invention is to provide a refractory raw material and a refractory raw material composition for obtaining such refractories. Still another object of the invention is to provide a process for producing graphite grains which can be used in them. A further object of the invention is to provide a process for producing the refractory raw material composition.